Landing an aircraft in zero-zero conditions requires, in effect, seeing through fog and rain. While there has not heretofore been a viable way to accomplish this, systems are being developed which take advantage of the latest advances in electronic imagery to make the aforementioned a reality.
One such system is designated as Synthetic Vision and incorporates an electronic image derived at least in the millimeter-wave frequency range, and in the infrared frequency range if necessary, and presents the image to a pilot in a head-up or head-down display. Such an arrangement will enable the pilot to effectively see through precipitation, particularly fog, for achieving safe day or night landings under zero-zero conditions. The background and history of the Synthetic Vision concept is described in an article entitled "Synthetic Vision Will Let Pilots See through Precip" by C. V. Glines and published in the August, 1990 issue of Professional Pilot.
A system of the type described requires an arrangement wherein a plan position indicator (PPI) cartesian map radar display is converted to a correctly scaled perspective display based on aircraft altitude, physical display dimensions and the viewing distance to a display screen.
The present applicant is aware of the following patents directed generally to perspective radar displays, all of which patents were issued to David W. Young and are classified in U.S. Class 343/5 LS: U.S. Pat. No. 3,866,218 which issued on Feb. 11, 1975; U.S. Pat. No. 3,866,222 which issued on Feb. 11, 1975; U.S. Pat. No. 3,896,432 which issued on Jul. 22, 1975; and U.S. Pat. No. 3,988,731 which issued on Oct. 26, 1976.
U.S. Pat. No. 3,866,218 relates to an arrangement wherein the horizontal position of echo signals displayed on a display screen are repositioned by operating adjusting means to shift the phase of a cyclic wave form by an amount to correct any phase shift between the output of a beam position sensor means and an actual azimuth position of the beam as a consequence of errors occurring in conversion by the beam position sensor means of the actual azimuth position of the beam and the generation of the cyclic wave form representing the actual position. The invention may advantageously be used with a perspective radar system wherein the generated beam is fan shaped in a vertical plane, but is acknowledged as being applicable to any scanning system wherein the beam is scanned back and forth through a given sector.
U.S. Pat. No. 3,866,222 relates to an arrangement for providing a perspective radar display on a radar cathode ray tube wherein a linear sweep is used so that the brightness of the picture is uniform throughout and the size of the display is not limited. Moreover, because the cathode ray tube includes a conventional linear sweep, it is adaptable for use with other input information normally requiring a linear sweep, as is the case in, for example, television.
U.S. Pat. No. 3,896,432 relates to a perspective radar system and a guidance control arrangement incorporating known information relative to an airport complex, including the range and heading of an aircraft to the airport runway from a given fixed point of the aircraft for comparisons with the perspective display provided by the perspective radar system.
U.S. Pat. No. 3,988,731 relates to altering a display provided by a perspective radar system to provide a display in perspective as would appear if the observer were at a different position from his actual position. The different position may be in altitude, in range, or a combination of both.
The present invention features providing a correctly scaled and registered perspective image in real-time, based on known constants and airframe supplied altitude data using exact derivations and no approximations. The arrangement disclosed completely refreshes all pixels in a display memory so that "fill" or "clean-up" operations are not needed. There are no Moire patterns due to partial fill.
Iterative implementation minimizes computational requirements. Divide operations are only required at the line rate and not the pixel rate for simplification purposes. Simple equations allow the use of a minimum number of common components and all out-of-range conditions are handled efficiently with data blanking provided when needed.
The above features of the invention which are seen to distinguish over the prior art noted above will be readily discerned from the detailed description of the invention which follows.